HEAVY METAL CONCENTRATIONS IN THE SOILS AND VEGETATION OF THE BÉKE-CAVE WATERSHED ( AGGTELEK-KARST , HUNGARY )

Our research took place on karstic area in Aggtelek National Park in Hungary. The heavy metal content of soils with three different texture and in the plants of the natural vegetation (oak, hornbeam-, corn leaves, greenery) were studied. Ratio of total (acid soluble) metal contents and bioavailable metal contents of the soils were calculated. Based on these results we determined the mobility of the metals in different soils. Used the metal contents of the soils and the vegetation we set up a sequence of the mobility of the metals between the soil and the most frequent plant species.


Introduction
This research has been complemented in recent years with studies on the heavy metal load in the soil-plant system.Contaminants in the soil including metals get into the system with the water infiltrating through the soil.In our test area, Aggtelek National Park anthropogenic impact has lately diminished, but traces of former pollution are still present, and contaminants can also enter from the atmosphere through dry and wet deposition from both domestic and transboundary sources.This was observed in the early 2000's (Szőke -Keveiné Bárány, 2003).
On karsts, as in other areas, the vegetation determines the organic matter content and the pH conditions of the underlying soil, which greatly affects the immobilization of metals.The vegetation of nature conservation areas is not harvested each year, so the majority of metals taken up by the plants build up in the soil over the years.If the metals are mobilized, they are returned to the soil, and through the infiltrating water, they contaminate the drinking water supplies of the karstic watersheds.The heavy metals could be present in soils in several forms.There are bound forms, not available for the plants.If the soil's nutrient retention capacity is analyzed, it is not enough to examine the "total" (acid-soluble) metal content of the soils, but we need to determine the amount plants are potentially able to uptake (= "bioavailable" metal content).
The present study examines the heavy metal uptake of plant samples (Sessile oak -Quercus petraea, cornel -Cornus sp., Hornbeam -Carpinus betulus, and the herbaceous layer as a mix) collected from the catchment area of the Béke Cave (Aggtelekkarst) concentrating on the amount of metal that different plants are able to extract from the soil.At the same time we also examined whether the "plant-extractable" metal element content extracted using EDTA solution actually meets the quantities taken up by the different plant species.

Showing of the studied area
The study was carried out in the catchment area of the Béke Cave (Aggtelekkarst), in an area of approx.10 km 2 (Fig. 1.).The surface development of the northern and southern part of the catchment was different.The northern part is formed mainly on well karstifiable Lower Triassic limestone bedrock with real rendzina soils, typical of true karsts.The southern part is a covered karst, where forest soils prevail, formed on Miocene and Pannonian sediments.The rendzina soils covering the surface are red clayey, brown and black rendzinas (Zámbó, 1971).In many places the limestone is covered by a relict red clay layer of variable depth (e.g. in the Red Lake area), which fills the bottom of the dolines, but also appears on the slopes, lower ridges, and rarely even on higher surfaces (Zámbó, 1970).On this layer red clayey and brown rendzina soils were developed with horizontal gradual transition.On the loose Tertiary sediments brown forest soils (brown forest soils with clay illuviation and Ramann brown soils, Luvisols) can be found.In addition, slope sediment soils, and small barren rocky surfaces also occur here Fig. 1.Aggtelek-Égerszög area's caves with stream and their catchment basin (dotted area = non karstic rocks; rectangle area = karstic rocks).The catchment of Béke cave is indicated with red line (Jakucs, 1985, Balázs, 1961) (Marosi -Somogyi, 1990;Stefanovits, 1996Stefanovits, , 1999;;Zámbó, 1998).
The potential vegetation are Turkey and downy oak-dominated forests (Quercetum petraae-pubescentis) and in the eastern part of the watershed sessile oak-hornbeam (Quercetum petraae -Carpinetum betulis) stands.The south-facing rocky slopes support rocky grasslands, ash-lime forests and slope steppes, with sub-montane alder groves along the surface streams and in some places planted coniferous stands.Crop production only occurs in areas adjacent to the settlements and on the border of the karstic and non-karstic areas (Marosi -Somogyi, 1990), mostly in the form of small arable lands.

Methods
98 soil samples were collected from different locations from a depth of 20-30 cm.This particular soil depth was chosen because the majority of the examined plants take up nutrients from this section of the soil profile.The plant samples were collected from individuals at the sampling locations.Of the woody plants, the mature leaves of the most common trees: oak, cornel and hornbeam were collected while samples from the mixed herbaceous vegetation were collected with mowing.The soil samples were dried and pulverized, the particle size distribution, pH and organic matter content were determined, and the heavy metal content was measured using two different methodologies.The "total" metal content was determined using a mix of nitric acid-hydrochloric acidperchloric acid, which takes into solution any type of compound except silicates.The granulometric composition of soil samples were determine according to elutriation method, with this end in view of Atterberg partice size categories.
The "plant-extractable" or "bioavailable" element concentration was determined using the Lakanen-Erviö method.The greenery samples were dried on room-temperature and they were milled.10 ml cc.HNO 3 was added to 1 g of plant sample, and held in a Gerhardt Kjeldatherm-type digester for 2-3 hours at a temperature of 120 °C, until the mixture became gelatinous.After cooling the sample under a fume hood, 3 cm 3 cc.HClO 4 was added and the mixture further heated for one hour at 120 °C.After cooling, the result was washed into a 50 cm 3 measuring flask (HS-08-1783-1:1983).The Cu, Zn, Ni, Co, Fe, Mn and Pb concentrations of the produced solution were determined using the ICP-OES method at the University of Veszprém, Department of Earth and Environmental Sciences.

Bioavailable heavy metal concentrations as a function of soil texture
Soils with different texture absorb heavy metals in a different way therefore we examined the ratio of the bioavailable heavy metal concentrations to the total according to soil texture properties (Table 1a., b.).Soil texture varies in the study area; the southern part is covered karst whereas in the northern open karst the heavy metal load can be significantly influenced by soil texture in the case of mobilization.
We can state that the ratio of "available" to "total" heavy metal content is highest in the Table 1a.The "total" and "bioavailable" heavy metal content of the soils (mg/kg) and their ratio (%) loamy soils for Cu, Ni and Zn.
Only a very small fraction of iron is present in the soil in a plant-available form, in all 3 texture types.For the other examined elements, this ratio is much higher.In the case of Mn and Pb the ratio is highest in clayey soils whereas for Co it is highest in the clayey loam soils.In clay and clayey loam soils 38-45% of copper is in an "available" form, in loamy textured soils its ratio is 70%.This high value is in accordance with data reported in the literature.According to Győri (1984) the total Cu content of the soils in Hungary is usually between 3.2-38 mg/kg while the bioavailable Cu content is between 4-20 mg/kg (Szabó et al., 1987).
Our results concerning the bioavailable Ni content in the area seem to be lower than in literature sources (Fig. 2.).According to Fekete (1988), Szabó (2000) the bioavailable Ni content in Hungary (based on the data of 6000 soils) is 4.43 mg/kg, however compared to the data of Bárány-Kevei et al. (2001) our measurements resulted in lower values.They latter source also mentions an available/ acid-extractable Ni ratio of 5-7% in samples from Aggtelek karst while the present results show higher values, especially in loam textured soils.
The total Pb content of the soils is lower than the results reported by Zseni (2001), also from the Aggtelek area (36-96 ppm).Her results for the plant extractable Pb (5-14 mg/kg) are higher, than those of Fekete (1988) who reports a mean of 6.43 mg/kg based on 6000 soil samples from Hungary.Our measurements show a higher available Pb content (32-34 %).The ratio of available/ Table 1b.The "total" and "bioavailable" heavy metal content of the soils (mg/kg) and their ratio (%) Fig. 2. The distribution of acid soluble (total) and EDTA extractable (bioavailable) Ni concentration on the studied area total lead content is similar to the results (35%) presented in the study of Szabó (2000).
The acid-soluble Zn content is between 87 and 110 mg/kg, which corresponds to the results of Bárány-Kevei et al. (2001), who examined soil samples from Aggtelek.There are differences between the "available" Zn content, since according to the already mentioned study of Szabó (2000), the plant extractable Zn content is between 3-10 ppm whereas our measurements gave a result of 6.5 mg/kg (in the present study, we measured 6-12 mg/kg).The bioavailable/total Zn ratio (6-16%) that we found is consistent with the findings of Szabó (2000) (12%).
The total amount of Co (Fig. 3.) is also similar to results from earlier literature (Szabó, 2000;Bárány-Kevei et al. 2001) as well as the bioavailable amount (1.7 to 4 mg/kg according to our findings), and the bioavailable/total ratio (literature 21-33%, own measurements 21-32 %).The total Mn content is 780 ppm, the bioavailable amount is 190-420 ppm and the ratio of them varies between 21-49%.
Based on the ratio of the amount of "available" heavy metals to the amount of the "total" heavy metal content, we defined the mobility order (using the means of the ratios in the lower soil layers): In clay soils: Pb>Mn>=Cu>Co>>Zn>Ni>Fe In clayey loam soils: Mn>Cu>Pb>Co>Zn>Ni>Fe In loam soils: Cu>Pb>Mn>Co>Zn>Ni>Fe The mobility order shows that iron (Fe) is present in the soils in the most bound form, followed by manganese, lead and copper.Cobalt, zinc and nickel are moderately mobilized (all 3 are in a similar situation in the mobility order).

Comparative analysis of the heavy metal content of soil and plant samples
At first we tried to define how the heavy metal content of the examined species answer compared to the total heavy metal content of the soil samples taken from the same location.To do this, we determined the value of the soil-plant transport coefficient.Then we calculated the ratio (%) of the heavy metal content of the plants and the "total" heavy metal content measured in the soil (Table 2.).The serial numbers of Table 2. mark the order calculated on the basis of the maximum values.The highest value of each metal in a particular plant according to soil type is indicated in italics.The table shows that the heavy metal content of the plants compared to the total heavy metal content of the soils is usually the highest in the case of loam textured soils.This suggests that loamy texture provide favorable conditions for the plants to uptake metal.Of the arboreal species the tendencies are quite clear for the hornbeam and cornel, whereas in the case of oak the value is in some cases highest in the other soil types.The Table 3. compares data from the literature with our own measurements.
Based on the soil-plant transport coefficient, we defined the mobility order of the examined heavy metals in the plants on the different soil types: Clay soils: Oak: Fe<Pb<Ni<Co<Zn=Cu<Mn  1% in any case.The lead, cobalt, and nickel values of 13, 31 and 44% respectively can be considered moderate.These are followed by zinc and copper with 54% and 76% content in the plants.The most mobile metal on the basis of the soil-plant system of transport coefficients is manganese.Plants are able to uptake and accumulate more of this metal than the soils' total content.The mobility order based on our results is different from those published in the literature (Kloeke et al., 1994;Szabó, 2000;Szabó, 2008).
Especially manganese shifts towards the end of the line in our study (towards better availability) especially in the case of the trees; the mobility order of cornel is similar to those in the literature.For herbaceous plants lead was found to be one of the most mobile elements (at the end of the mobility order) for all soil types.

Conclusion
In the present study we showed that in karst areas the ratio of plant-extractable ("available") to acid-soluble ("total") heavy metal content differs according to soil texture.In the loam soils (situated in the southern part of the study area, on the covered karst) copper, nickel and zinc show the highest available/total ratio, in the clay soils (mainly occurring in the open karst area) manganese and lead, whereas in clayey loam soils iron and cobalt can be found in the most available form.The examination of the metal content of some plant species showed that in the case of hornbeam, cornel and the mixed herbaceous layer most of the examined heavy metals can be found in the highest concentrations on the loam soils, except for cobalt.In the case of hornbeam the mobility order based on the soil-plant transport coefficient is the same for each of the three soil types.For the arboreal species (oak, hornbeam) the soil-plant transport coefficient of manganese is behind in the mobility order while those of cornel and the mixed herbaceous layer are in the middle.The herbaceous layer takes up lead in high proportions on all three soil texture types.This indicates that lead exposure on the karst must be prevented by all means, due to the risk presented by high mobility in these areas.

Fig. 3 .
Fig. 3.The distribution of acid soluble (total) and EDTA extractable (bioavailable) Co concentration on the studied area

Table 2 .
The ratio of the heavy metal content of the plants and the "total" heavy metal content measured in the soil (%)

Table 3 .
The transport coefficients of heavy metals in the soil-plant system based on literature data and own results